1. Field of the Invention
The present invention relates to an optical pickup apparatus for optically reproducing recorded signals in a recording medium. In the apparatus, a beam of light is projected onto the recording medium and the light transmitted through or reflected from the recording medium is divided into two light components. The two components are photoelectrically converted into two signals and the difference between the two signals is detected to reproduced the recorded signal.
2. Related Background Art
In the art, many methods have been known and used for the reproduction of the recorded signals.
Among them, the above-mentioned optical method has particular advantages.
According to the method, a beam of light is projected onto the recording medium and the light transmitted through or reflected from the recording medium is divided into two beams which are then photoelectrically converted into two signals. The difference between the two signals is detected to reproduce the signal recorded in the recording medium. Obviously, this optical method is easy to carry out and it enables removing any in-phase noise in a very simple manner.
For these advantages, the optical signal-reproducing method has been applied to those pickup apparatus in which signals are read out from a magneto-optical recording disc according to the principle of the Kerr effect or the Faraday effect. An example of the prior art optical pickup apparatus has been disclosed in U.S. Pat. No. 4,599,714.
In reproducing signals according to the above optical method, noise, such as disc noise and laser noise may be generated by any damage on the magneto optical recording disc, any irregularity in the laser light source etc. Such noise is in-phase noise which is generated before the splitting of the beam and equally introduced into the split beams. According to the optical method, as shown in the above-referred patent specification, this in phase noise can be removed in a simple manner.
FIG. 1 shows, in sectional view, the construction of the main part of the prior art pickup apparatus for magnetooptical recording disc.
In FIG. 1, the reference numeral 5 denotes a recording medium in which information has been recorded. The apparatus for reproducing the recorded signal comprises a laser diode 1, a collimator lens 2, a beam splitter 3, an objective lens 4, a polarized beam splitter 16 and first and second photodetectors 11 and 12. Denoted by 17 and 18 are printed substrates. The laser diode 1 generates a linearly polarized light beam.
In the prior art apparatus, the reproducing light from the magnetooptical disc 5 is divided into two beams by the polarized beam splitter 16 whose splitting surface is disposed 45.degree. inclined relative to the incident light. The first and second photodetectors 11 and 12 on separate substrates are positioned to receive the split beams by the splitter. An important problem involved in the prior art apparatus is the precise positioning of the polarized beam splitter 16 relative to the two separate photodetectors 11 and 12. A particular structure is required, for the positioning and a particular positioning operation is needed.
The number of working steps needed for the apparatus is undesirably increased by the positioning. In addition, wiring is required to electrically connect the first photodetector with the first substrate and also the second photodetector with the second substrate.